Method and apparatus for controlling current draw while charging a battery array

ABSTRACT

An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of the charge control system for charging a battery unit includes a current sink switchingly coupled with the input locus for selectively contributing a predetermined current draw at the input locus. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit includes the steps of: (a) providing a current sink switchingly coupled with the input locus; (b) sensing at least one predetermined condition in the battery unit; and (c) switchingly engaging the current sink when the at least one predetermined condition satisfies at least one predetermined criteria.

[0001] This is a continuation of application U.S. Ser. No. 10/159,138,filed May 30, 2002.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to battery charging methods andapparatuses, and especially to charging battery arrays having aplurality of battery cells. In particular, the present invention isuseful in connection with balancing individual cells while chargingmultiple cell battery arrays, including Lithium-ion or Lithium polymerbattery packs.

[0003] Many systems use batteries that are configured as battery packsor arrays including a plurality of individual battery cells coupled inseries. Such a configuration is commonly encountered, for example insystems that need to maximize run time and use Li-ion (Lithium ion) orLi-polymer chemistry. The battery arrays may include two cells (e.g.,for consumer products such as camcorders or cameras) up to four or morecells (e.g., for high-end notebook computers). In multi-cell batteryarrays such as Li-ion battery packs with cells arranged in series theoverall battery pack coulombmetric capacity is limited by the leastcapacity cell. As a result, energy capacity of a battery pack isdependent upon how closely individual cell voltages are matched. Cellmismatches of 100 mv (millivolts) can decrease battery pack energycapacity by more than 10%.

[0004] Such cell mismatches can be introduced during fabrication orduring the processes of charging and discharging the battery array. Cellfactory manufacturing can be as closely controlled as to producecapacity differences among cells in a battery array within 50 mv forLi-ion cells. However, cell imbalance or mismatch may be introduced by anumber of factors independent of initial factory matching. Those factorscontributing to cell imbalance include, by way of example, variations inindividual cell chemistry, cell impedance, self discharge rates,capacity fade, operating temperature and other variations amongrespective individual cells. Cell temperature mismatches are asignificant cause of cell mismatching that is a relatively common traitfor densely packed products having multiple individual heat sourceslocated close to the battery pack. For example, a 20° C. temperaturemismatch can cause a voltage differential among cells as high as 100 mvin a charge cycle. One example of such a product is a notebook computer.

[0005] Because of the various problems resulting from cell mismatches,cell balancing while charging a battery pack is an important factor inmaximizing battery pack energy capacity. Two methods are currently usedto balance cells during charging battery packs having multiple cells.

[0006] One method presently in use involves differential cellmeasurement. Using differential cell measurement, individual cellvoltages are sampled and differential cell voltages are calculatedduring charging. When a high differential voltage is detected, chargingis interrupted and individual cells are selectively dischargedappropriately to obtain balance among cells. This differential cellmeasurement approach facilitates accurate cell balancing, but complexcircuitry and methodology are required to practice the method. Forexample, it is necessary to use cell voltage translation, A/D(analog-to-digital) conversion and multiple arithmetic operations topractice differential cell measurement. Because of the complexity of theequipment and calculations required for practicing the method,differential cell measurement is usually found to be employed for cellbalancing in high-end, high-cost products that include an analogfront-end IC (integrated circuit) for measuring voltages in cooperationwith a microcontroller or CPU (central processing unit)—based evaluatingsystem.

[0007] A second method presently used to balance cells during chargingbattery packs having multiple cells is a ground referenced, fixedthreshold method. Using such a fixed threshold method, when one cellreaches a first predetermined threshold it is discharged to a lowersecond threshold. The second threshold is usually a fixed threshold setto a voltage equal to or greater than the target voltage, or regulatedvoltage for the battery pack. The fixed threshold method is lessexpensive to equip and practice than the differential cell measurementmethod described above, but it can suffer from low accuracy and canrequire significantly longer charge times than may be experienced usinga differential cell measurement method described above. The likelihoodfor longer charge times is particularly high if initial voltage mismatchamong cells is large.

[0008] In both the differential cell measurement method and the fixedthreshold method, a charger on the system side must be controlled by thehost product it is charging in the battery pack in order to preventfalse termination of charging during sampling or cell balancingintervals.

[0009] There is a need for a low cost accurate cell balancing methodthat does not significantly lengthen charge times.

[0010] There is a particular need for such a low cost cell balancingmethod that does not require control by or communication with the hostdevice being charged.

[0011] There is also a need for a charge control apparatus that canoperate without requiring control by or communication with the hostdevice in the battery pack being charged and can permit cell balancingwithout causing false termination of charging operations.

SUMMARY OF THE INVENTION

[0012] A method for controlling voltage levels among cells whilecharging a battery array to a target voltage uses cell balancing modesemploying respective charging currents. The method includes the stepsof: (a) In no particular order: (1) establishing a parametric criterionfor identifying each cell balancing mode; (2) identifying a performanceparameter associated with selected cells for each cell balancing mode;and (3) establishing an exit criterion for each cell balancing mode; (b)ascertaining the onset of charging; (c) identifying the extant cellbalancing mode; (d) employing the charging current for the extant cellbalancing mode for cell balancing; (e) for selected cells: (1) obtainingan extant parameter associated with the cell; (2) comparing each extantparameter with the exit criterion; and (3) repeating steps (e)(1)through (e)(2) until the extant parameter satisfies the exit criterion;(f) terminating the extant cell balancing mode; (g) repeating steps (c)through (f) until the target voltage is achieved; (h) terminatingcharging.

[0013] It is, therefore, an object of the present invention to provide asystem and method for controlling cell balancing of a battery array thatis inexpensive, accurate and does not significantly lengthen chargetimes.

[0014] It is a further object of the present invention to provide asystem and method for controlling cell balancing of a battery array thatcan permit cell balancing without causing false termination of chargingoperations.

[0015] Further objects and features of the present invention will beapparent from the following specification and claims when considered inconnection with the accompanying drawings, in which like elements arelabeled using like reference numerals in the various figures,illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a graphic diagram of a representative charging evolutionillustrating charging current and cell voltage as they relate to time,and providing an overview of the method of the present invention.

[0017]FIG. 2 is a schematic diagram illustrating representative cellbalancing steps employed in carrying out the method of the presentinvention.

[0018]FIG. 3 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for cell balancing a battery array in a first cell balancingmode as illustrated in FIG. 1.

[0019]FIG. 4 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for balancing cells in a battery array in a second chargingmode as illustrated in FIG. 1.

[0020]FIG. 5 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for cell balancing a battery array in a third cell balancingmode as illustrated in FIG. 1.

[0021]FIG. 6 is a flow diagram illustrating the preferred embodiment ofthe method of the present invention.

[0022]FIG. 7 is an electrical schematic diagram illustrating a firstembodiment of the charging apparatus of the present invention.

[0023]FIG. 8 is an electrical schematic diagram illustrating a secondembodiment of the charging apparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024]FIG. 1 is a graphic diagram of a representative charging evolutionillustrating charging current and cell voltage as they relate to time,and providing an overview of the method of the present invention. InFIG. 1, a graphic plot 10 is presented with a first axis 12 representingcharging current/cell voltage, and a second axis 14 representing time. Acurve 16 represents cell voltage (for a representative cell in a multicell battery array) as a function of time. A curve 18 representscharging current as a function of time. Curve 18 has a taper section 19representing the portion of the charge cycle during which chargingcurrent markedly decreases while cell voltage markedly increases. Twotaper sections 19, 19 a represent such a tapering effect for tworepresentative cells in a battery array.

[0025] Graphic plot 10 is segmented into three regions: Region 1, Region2, and Region 3. Region 1 spans a time interval from time to t₀ time t₁.Region 2 spans a time interval from time t₁ to time t₂. Region 3 spanstimes from time t₂ to later times. Region 1 represents a portion of acharging cycle during which the battery array is substantially depletedand has very little or no charge. In Region 1 cell dV/dQ (change ofvoltage per change of charge) is at a maximum, cell voltage (curve 16)is rising significantly and charging current (curve 18) is at a minimumlevel. Region 2 represents a portion of a charging cycle during whichthe battery array has a medium charge level. In Region 2 cell dV/dQ isminimal, cell voltage (curve 16) rises slightly to moderately andcharging current (curve 18) is at a maximum level. Region 2 is the maincharge time duration of the representative charge cycle of a batteryarray represented by graphic plot 10. Region 3 represents a portion of acharging cycle during which the battery array has a high charge level.In Region 3, cell dV/dQ is at a maximum, and charging current (curve 18)begins to decrease as soon as a cell reaches its respective targetvoltage (or regulation voltage; or regulated voltage). Such a decreaseis indicated in FIG. 1 in taper section 19 for one cell of the batteryarray being charged, and is indicated in taper section 19 a for a secondcell of the battery array being charged.

[0026] The respective durations of Region 1 (interval t₀-t₁), Region 2(interval t₁-t₂) and Region 3 (interval t₂ to later times) depend uponthe charge level of the battery (or battery pack) being charged. Thus,one can lengthen the duration of a charge operation, or the interval ofRegion 1, Region 2 or Region 3, by varying the charge state of thebattery pack being charged. Adjustment of respective charge states ofvarious battery cells in a battery pack is employed to achieve balancingamong respective battery cells, but may also lengthen the overalltime-to-charge experienced in bringing the battery pack to a fullycharged state.

[0027] A common approach to balanced charging involves performing cellsampling during charging to ascertain which cells are mismatched, anddischarging selected cells to reduce the cell mismatches that areidentified during sampling. Carrying out such cell sampling anddischarge operations while applying full charge current to the batteryarray (e.g., in Region 2; FIG. 1) does not provide effective cellbalancing. This is so because, the voltage measured across a cellincludes a voltage portion contributed by the impedance of the cellbeing evaluated, plus the intrinsic cell voltage. Impedance mismatchesamong individual cells are common and those mismatches will contributeto errors in measured voltages for cells. A better practice that yieldsmore accurate cell voltage measurements is to perform cell sampling anddischarge operations after suspending or interrupting the chargecurrent. Thus, the charge operation is interrupted for a period topermit sampling of cells and selective discharge among cells to reducemismatches discovered during sampling. Such a suspension of chargingeliminates cell impedance contribution to cell voltage measurements andyields more accurate indications of cell mismatches. A problem arises incharging systems in which a charger is not aware that a charge isintentionally interrupted, for example in systems in which the hostdevice being charged does not communicate with the charger. In such asituation the charger may detect a taper current (e.g., taper sections19, 19 a; FIG. 1) or a battery open condition which will cause thecharger to prematurely and erroneously terminate the charge.

[0028]FIG. 1 illustrates the relationships among charging current,charge voltage and time in charging a battery. The delineation of Region1, Region 2 and Region 3 in terms of time intervals presumes that, forexample, one uninterruptedly applies a charging current to a battery tocharge the battery. The advantage provided by the method of the presentinvention is illustrated in overview in FIG. 1. An important feature ofthe method of the present invention is to effect cell balancing amongcells in a battery array before proceeding from one of Region 1, Region2, Region 3 to another of Region 1, Region 2, Region 3. That is, thedriving impetus in practicing the preferred embodiment of the method ofthe present invention is not time nor is it charging current. Thedriving impetus in practicing the preferred embodiment of the method ofthe present invention is to achieve a predetermined degree of balanceamong cells in the battery array in one of Region 1, Region. 2, Region 3before proceeding to the next region of Region 1, Region 2, Region 3.

[0029] Thus, according to the preferred embodiment of the method of thepresent invention there are thresholds established for use indetermining whether sufficient balance is achieved among cells beforeproceeding to a next region of Region 1, Region 2, Region 3. Thethresholds are preferably voltage thresholds that are treated using acell balancing algorithm for determining whether desired balance isachieved among cells. The present invention contemplates that differentcell balancing algorithms may be employed in different regions foreffecting the desired cell balancing.

[0030] In Region 1, a lower Region 1 voltage threshold R1THL and ahigher Region 1 voltage threshold R1THH are established. In Region 2, alower Region 2 voltage threshold R2THL and a higher Region 2 voltagethreshold R2THH are established. In Region 3, as will be described ingreater detail later in connection with FIG. 5, voltage thresholds maybe dynamically shifted in order to come closer to achieving regulatedvoltage for all cells in the battery array. Thus, in Region 3 a firstlower Region 3 voltage threshold R3THL1 and a first higher Region 3voltage threshold R3THH1 are established. Region 3 voltage thresholdsR3THL1, R3THH1 can be iteratively dynamically shifted during charging aplurality of times, as indicated by there also being established an nthlower Region 3 voltage threshold R3THLn and an nth higher Region 3voltage threshold R3THHn. The use of the term “n” is intended toindicate that there is no particular limit to the number of iterativeestablishings for Region 3 voltage thresholds R3THLn, R3THHn inpracticing the method of the present invention. Of course, one mayconsider that time of charge is a parameter of importance. In such asituation, one may limit the number of iterative establishings forRegion 3 voltage thresholds R3THLn, R3THHn by some means, such aslimiting “n” to a predetermined number, by providing a time limit formoving on to a subsequent method step, or by another means.

[0031] The method of the present invention provides that one satisfypredetermined criteria for selected cells of the battery array(preferably, for all cells in the battery array) vis-à-vis Region 1voltage thresholds R1THL, R1THH before selecting a cell balancing method(curve 18; FIG. 1) to operate in Region 2. The cell balancing algorithmthat is employed to seek cell balancing effectively controls the loadseen by the charger. This effectively controlling of load seen by thecharger is carried out by effecting slower charging of selected cellswhile permitting other cells to continue to be charged at a highercharge rate. That is, the cell balancing algorithm configures the loadseen by the charger in a manner that causes the charger to continue tosee a state of charge in the battery cell array that is to be treatedusing charging current as it is applied in Region 1 or to see a state ofcharge in the battery cell array that is to be treated using chargingcurrent as it is applied in Region 2.

[0032] Similarly, one employs a cell balancing algorithm (notnecessarily the same cell balancing algorithm as is used in connectionwith Region 1) to achieve predetermined criteria for selected cells ofthe battery array (preferably, for all cells in the battery array)vis-à-vis Region 2 voltage thresholds R2THL, R2THH before selecting acell balancing method to operate in Region 3. Further, one employs acell balancing algorithm (not necessarily the same cell balancingalgorithm as is used in connection with Region 1 or Region 2) to achievepredetermined criteria for selected cells of the battery array(preferably, for all cells in the battery array) vis-à-vis Region 3voltage thresholds R2THLn, R3THHn before terminating the cell balancing

[0033]FIG. 2 is a schematic diagram illustrating representative chargingsteps employed in carrying out the method of the present invention. InFIG. 2, a time line 30 illustrates occurrence in time of steps or modes32 in a charging process, and variation in time of a voltage samplingsignal 34. The charging process illustrated in FIG. 2 is engaged in acharge step from before the earliest time illustrated in FIG. 2 until atime t₁. At time t₁ the charge process enters a relax step. During therelax step the charger is not applying fast charge current (curve 18;FIG. 1) to the battery array in order that any charging current maydampen to substantially zero. In such manner, one avoids introducingerror into measuring cell mismatches from the charging currenttraversing respective cell impedance, as discussed earlier in connectionwith FIG. 1. Voltage sampling signal 34 is pulsed during the intervalt₂-t₃ for a sampling interval having a duration t_(s), effectingsampling of cell voltage at time t₃. At time t₃, with cell samplingcomplete, the charge process enters a cell balancing step. The cellbalancing step occupies a time interval t₃-t₁₀. Voltage sampling signal34 is pulsed during intervals t₄-t₅, t₆-t₇, t₈-t₉ (each interval havinga duration t_(s)) to effect voltage sampling in support of the cellbalancing step then extant at times t₅, t₇, t₉. The charge processenters another step at time t₁₀, preferably another charge step.

[0034]FIG. 3 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for charging a battery array in a first charging mode asillustrated in FIG. 1. FIG. 3 is a representation of a preferredapplication of the method of the present invention in Region 1 (FIG. 1).In FIG. 3, voltages in three cells in a battery array are illustrated asthey vary with respect to time. A first cell (Cell 1) exhibits a cellvoltage V₁, a second cell (Cell 2) exhibits a cell voltage V₂ and athird cell (Cell 3) exhibits a cell voltage V₃ over a time intervalt1₁-t1₁₀. None of cells Cell 1, Cell 2, Cell 3 are shown in FIG. 3.

[0035] The preferred embodiment of the method of the present inventionbegins with, in no particular order, establishing a parametric criterionfor identifying the respective cell balancing mode, identifying at leastone performance parameter associated with the cells and establishing anexit criterion related to the performance parameter. In the situationillustrated in FIG. 3, the performance parameter is established as alower cell voltage threshold V1th_(LOW) and an upper cell voltagethreshold V1th_(HIGH). The charge process is identified as being inRegion 1 (FIG. 1) by cell voltages V₁, V₂, V₃ being less than upper cellvoltage threshold V1th_(HIGH). A voltage sampling signal (e.g., voltagesampling signal 34; FIG. 2) periodically samples cell voltages V₁, V₂,V₃ during sample intervals of duration t_(s) effective at times t1₂,t1₄, t1₆, t1₈, t1₁₀. In the exemplary application of the method of thepresent invention illustrated in FIG. 3, at the end of each samplingperiod t_(s) a decision is made which cells should be charged moreslowly: any cell having a cell voltage greater than lower cell voltagethreshold V1th_(LOW) is charged at a slower rate in order to reducemismatch among cells. One structure for effecting selective slowercharging among cells is to provide selectively established current pathsfor rerouting a portion of current around a selected cell. If there isno charging current applied when the circuit is configured to redirectcurrent around selected cells, then the selected cells will discharge.Thus, none of the cells (Cell 1, Cell 2, Cell 3) are candidates forslower charging at sampling times t1₂, t1₄. At sampling time t1₆ cellvoltages V₂, V₃ are both above lower cell voltage threshold V1th_(LOW)and therefore cells Cell 2, Cell 3 are charged more slowly, as isindicated by the lesser slope of curves representing cell voltages V₂,V₃ from sampling time t1₆ onward to time t1₁₀. Cell voltage V₁ is notdetected as exceeding lower cell voltage threshold V1th_(LOW) untilsampling time t1₈, after which time cell Cell 1 is charged more slowly,as is indicated by the lesser slope of the curve representing cellvoltage V₁ from sampling time t1₈ onward to time t1₁₀.

[0036] A representative exit criterion for the charge mode illustratedin FIG. 3 provides that Region 1 may be exited when any cell voltage V₁,V₂, V₃ exceeds upper cell voltage threshold V1th_(HIGH). At sample timet1₁₀ cell voltage V₃ exceeds upper cell voltage threshold V1th_(HIGH),so the cell balancing operation illustrated in FIG. 3 ceases andcharging continues. Using this representative exit criteria it ispossible that the cell balancing operation may cease before completebalancing is achieved among battery cells. This may be permitted inorder to avoid too much delay in charging operations. Alternate exitcriteria, or an alternate charge balancing algorithm or a combination ofalternate exit criteria and an alternate charge balancing algorithm maybe substituted for the representative criteria and balancing algorithmdiscussed here in order to assure that complete cell balancing isachieved before permitting the cell balancing operation to cease.Alternatively, a time limit may be imposed to establish a time outperiod as an additional exit criterion. In such a case, if the time outperiod expires, the cell balancing operation ceases regardless ofwhether any cell voltage V₁, V₂, V₃ has exceeded upper cell voltagethreshold V1th_(HIGH).

[0037]FIG. 4 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for charging a battery array in a second charging mode asillustrated in FIG. 1. FIG. 4 is a representation of a preferredapplication of the method of the present invention in Region 2 (FIG. 1).In FIG. 4, voltages in three cells in a battery array are illustrated asthey vary with respect to time. A first cell (Cell 1) exhibits a cellvoltage V₁, a second cell (Cell 2) exhibits a cell voltage V₂ and athird cell (Cell 3) exhibits a cell voltage V₃ over a time intervalt2₁-t2₁₀. None of cells Cell 1, Cell 2, Cell 3 are shown in FIG. 4.

[0038] The preferred embodiment of the method of the present inventionbegins with, in no particular order, establishing a parametric criterionfor identifying the respective cell balancing mode, identifying at leastone performance parameter associated with the cells and establishing anexit criterion related to the performance parameter. In the situationillustrated in FIG. 4, the performance parameter is established as alower cell voltage threshold V2th_(LOW) and an upper cell voltagethreshold V2th_(HIGH). The charge process is identified as being inRegion 2 (FIG. 1) by cell voltages V₁, V₂, V₃ being greater than uppercell voltage threshold V1th_(HIGH) (FIG. 3). A voltage sampling signal(e.g., voltage sampling signal 34; FIG. 2) periodically samples cellvoltages V₁, V₂, V₃ during sample intervals of duration t_(s) effectiveat times t2₂, t2₄, t2₆, t2₈, t2₁₀. In the exemplary application of themethod of the present invention illustrated in FIG. 4, at the end ofeach sampling period t_(s) a decision is made which cells should becharged more slowly: any cell having a cell voltage greater than theaverage cell voltage V_(AVG) is charged more slowly in order to reducemismatch among cells. One structure for effecting selective slowercharging among cells is to provide selectively established current pathsfor rerouting a portion of current around a selected cell. If there isno charging current applied when the circuit is configured to redirectcurrent around selected cells, then the selected cells will discharge.Average cell voltage V_(AVG) is calculated:$V_{AVG} = \frac{V_{1} + V_{2} + {\ldots \quad V_{n}}}{N}$

[0039] Where N=Number of Cells in Array

[0040] In the charge operation illustrated in FIG. 4, N=3. None of thecells (Cell 1, Cell 2, Cell 3) are candidates for slower charging atsampling time t2₂. At sampling time t2₄ cell voltages V₂, V₃ are bothabove average cell voltage threshold V_(AVG) and therefore cells Cell 2,Cell 3 are charged more slowly, as is indicated by the lesser slope ofcurves representing cell voltages V₂, V₃ from sampling time t2₄ onwardto time t2₆.

[0041] At sampling time t2₆ cell voltage V₂ is equal with averagevoltage V_(AVG) and therefore cell Cell 2 commences charging at a fasterrate, as indicated by the increased slope of the curve representing cellvoltage V₂. Cell voltage V₃ remains higher than average voltage V_(AVG)at sampling time t2₆, and therefore cell Cell 3 continues to be chargedmore slowly. At sampling time t2₈ cell voltages V₂, V₃ are both higherthan average voltage V_(AVG). Thus cell Cell 3 continues to be chargedmore slowly, and cell Cell 2 again commences charging more slowly. Cellvoltage V₁ is not detected as exceeding average cell voltage V_(AVG) andcell Cell 1 is therefore not affected. A representative exit criterionfor the charge operation illustrated in FIG. 4 provides that Region 2may be exited when all cell voltages V₁, V₂, V₃ exceed upper cellvoltage threshold V2th_(HIGH). At sample time t2₁₀ none of cell voltagesV₁, V₂, V₃ exceeds upper cell voltage threshold V2th_(HIGH), so the cellbalancing operation illustrated in FIG. 4 continues. The load seen bythe charger is kept at a current level that indicates to the chargerthat cell balancing operations in Region 2 should continue, and thecharge operation is longer than it would have been without cellbalancing in Region 2. This is an example of the dynamic nature of themethod of the present invention by which transition from one chargingmode (e.g., Region 1, Region 2, Region 3; FIG. 1) to another chargingmode is not driven by time but rather by a real-time evaluation ofconditions in the cells of the array.

[0042] Using this representative exit criteria it is possible that thecell balancing operation may cease before complete balancing is achievedamong battery cells. This may be permitted in order to avoid too muchdelay in charging operations. Alternate exit criteria, or an alternatecharge balancing algorithm or a combination of alternate exit criteriaand an alternate charge balancing algorithm may be substituted for therepresentative criteria and balancing algorithm discussed here in orderto assure that complete cell balancing is achieved before permitting thecell balancing operation to cease. Alternatively, a time limit may beimposed to establish a time out period as an additional exit criterion.In such a case, if the time out period expires, the cell balancingoperation ceases regardless of whether all cell voltages V₁, V₂, V₃ haveexceeded upper cell voltage threshold V2th_(HIGH).

[0043]FIG. 5 is a graphic diagram illustrating cell voltage as afunction of time in a representative practicing of the method of presentinvention for charging a battery array in a third charging mode asillustrated in FIG. 1. FIG. 5 is a representation of a preferredapplication of the method of the present invention in Region 3 (FIG. 1).In FIG. 5, voltages in three cells in a battery array are illustrated asthey vary with respect to time. A first cell (Cell 1) exhibits a cellvoltage V₁, a second cell (Cell 2) exhibits a cell voltage V₂ and athird cell (Cell 3) exhibits a cell voltage V₃ over a time intervalt3₁-t3₁₄. None of cells Cell 1, Cell 2, Cell 3 are shown in FIG. 5.

[0044] The preferred embodiment of the method of the present inventionbegins with, in no particular order, establishing a parametric criterionfor identifying the respective cell balancing mode, identifying at leastone performance parameter associated with the cells and establishing anexit criterion related to the performance parameter. In the situationillustrated in FIG. 5, the performance parameter is initiallyestablished as a first lower cell voltage threshold V3th_(LOW1) and afirst upper cell voltage threshold V3th_(HIGH1). The charge process isidentified as being in Region 3 (FIG. 1) because all cell voltages V₁,V₂, V₃ are greater than upper cell voltage threshold V2th_(HIGH) (FIG.4). A voltage sampling signal (e.g., voltage sampling signal 34; FIG. 2)periodically samples cell voltages V₁, V₂, V₃ during sample intervals ofduration t_(s) effective at times t3₂, t3₄, t3₆, t3₈, t3₁₀, t3₁₂, t3₁₄.In the exemplary application of the method of the present inventionillustrated in FIG. 5, at the end of each sampling period t_(s) certaindecisions are made regarding treatment of various cells. A two-stepevaluation is performed: (1) If any cell voltage V_(n) is greater thanthe then extant upper cell voltage threshold V3th_(HIGHn) then thethresholds V3th_(LOWn), V3th_(HIGHn) are shifted higher to newthresholds V3th_(LOWnew), V3th_(HIGHnew) at levels where all cellvoltages V_(n) are less than new upper cell voltage thresholdV3th_(HIGHnew). Further evaluation is effected with respect to the newhigher thresholds V3th_(LOWnew), V3th_(HIGHnew). If no cell voltageV_(n) is greater than the then extant upper cell voltage thresholdV3th_(HIGHnew) then the thresholds V3th_(LOWnew), V3th_(HIGHnew) remainat their then extant levels and further evaluation is effected withrespect to the unchanged thresholds. (2) If a respective cell voltageV_(n) is greater than the then extant lower cell voltage thresholdV3th_(LOWnew) (after the threshold evaluations have been made pursuantto step (1) above) then the respective cell Cell n displaying thevoltage V_(n) is charged more slowly. One structure for effectingselective slower charging among cells is to provide selectivelyestablished current paths for rerouting a portion of current around aselected cell. If there is no charging current applied when the circuitis configured to redirect current around selected cells, then theselected cells will discharge. A further evaluation is also employed:(3) If all cells Cell 1, Cell 2, Cell 3 have cell voltages V₁, V₂, V₃between then extant thresholds V3th_(LOWn), V3th_(HIGHn) it is assumedthat cells Cell 1, Cell 2, Cell 3 are balanced. Under such assumedbalanced circumstances (a) a full charge is effected to charge all ofcells Cell 1, Cell 2, Cell 3 as fast as possible; and (b) thresholdsV3th_(LOWn), V3th_(HIGHn) are shifted higher to new thresholds at levelswhere all cell voltages V_(n) are less than new upper cell voltagethreshold V3th_(HIGHnew). Further, if any of the cell voltages V₁, V₂,V₃ is between then extant thresholds V3th_(LOWn), V3th_(HIGHn) andanother cell voltage V₁, V₂, V₃ remains below lower extant thresholdV3th_(LOWn), then the cell voltage V₁, V₂, V₃ that is between thenextant thresholds V3th_(LOWn), V3th_(HIGHn) is discharged (for example,no charging current is applied to the affected cell and an alternatecurrent path redirecting circuitry around the affected cell is enabledor established), and the extant charge is maintained on the cells havinga cell voltage V₁, V₂, V₃ below then extant lower threshold V3th_(LOWn).Still further, if all cell voltages V₁, V₂, V₃ are below lower extantthreshold V3th_(LOWn), then all cell voltages V₁, V₂, V₃ are charged atthe fast charge rate together.

[0045] At sampling time t3₂, cell voltage V₃ is greater than the thenextant upper cell voltage threshold V3th_(HIGH1) so thresholdsV3th_(LOW1), V3th_(HIGH1) are shifted to higher levels sufficiently toestablish new thresholds V3th_(LOW2), V3th_(HIGH2) at levels where allcell voltages V₁, V₂, V₃ are less than new upper cell voltage thresholdV3th_(HIGH2). Then cell voltages V₁, V₂ are below the then extant lowercell voltage threshold V3th_(LOW2) so that no slower charging iseffected for cells Cell 1, Cell 2 having cell voltages V₁, V₂.

[0046] At sampling time t3₄ cell voltage V₃ is greater than extant lowercell voltage threshold V3th_(LOW2), but cell voltages V₂, V₃ are belowlower voltage threshold V3th_(LOW2), so cell Cell 3 is discharged andcells Cell 2, Cell 3 are maintained at their respective voltage levels,as manifested in FIG. 5 by a downward slope for the curve representingcell voltage V₃ from sampling time t3₄ onward. At sampling time t3₄ cellvoltages V₁, V₂ are both below lower cell voltage threshold V3th_(LOW2)so the then extant charge levels on cells Cell 1, Cell 2 are maintained,as manifested by the flat voltage levels for the curves representingcell voltages V₂, V₃ from sampling time t3₄ onward.

[0047] At sampling time t3₆ cell voltages V₁, V₂, V₃ remain below lowercell voltage threshold V3th_(LOW2), so that all of cells Cell 1, Cell 2,Cell 3 are charged at the fast charge rate, as is indicated by theparallel curves representing cell voltages V₁, V₂, V₃ from sampling timet3₆ onward.

[0048] At sampling time t3₈ all cell voltages V₁, V₂, V₃ are betweenthresholds V3th_(LOW2), V3th_(HIGH2) so thresholds V3th_(LOW2),V3th_(HIGH2) are shifted to higher levels sufficiently to establish newthresholds V3th_(LOW3), V3th_(HIGH3) at levels where all cell voltagesV₁, V₂, V₃ are less than new upper cell voltage threshold V3th_(HIGH3).Cells Cell 1, Cell 2, Cell 3 continue at full charge.

[0049] At sampling time t3₁₀ all cell voltages V₁, V₂, V₃ are below thenextant lower cell voltage threshold V3th_(LOW3) and cells Cell 1, Cell2, Cell 3 continue at full charge.

[0050] At sampling time t3₁₂ all cell voltages V₁, V₂, V₃ are betweenthresholds V3th_(LOW3), V3th_(HIGH3) so thresholds V3th_(LOW3),V3th_(HIGH3) are shifted to higher levels sufficiently to establish newthresholds V3th_(LOW4), V3th_(HIGH4) at levels where all cell voltagesV₁, V₂, V₃ are less than new upper cell voltage threshold V3th_(HIGH4).Cells Cell 1, Cell 2, Cell 3 continue at full charge.

[0051] At a time between sampling time t3₁₂ and time t3₁₃ all cellvoltages V₁, V₂, V₃ achieve regulated voltage V_(REG) (also knowvariously as rated voltage or target voltage). The method permitsexiting or terminating cell balancing operations when an extant uppercell voltage threshold V3th_(HIGHn) exceeds V_(REG). That situation ispresent in FIG. 5 during the interval t3₁₂-t3₁₃. Accordingly, cellbalancing is terminated at sampling time t3₁₄. Alternatively, a timelimit may be imposed to establish a time out period as an additionalexit criterion. In such a case, if the time out period expires, the cellbalancing operation ceases in Region 3 and cell balancing is terminatedregardless of whether an extant upper cell voltage thresholdV3th_(HIGHn) exceeds V_(REG).

[0052]FIG. 6 is a flow diagram illustrating the preferred embodiment ofthe method of the present invention. In FIG. 6, a method 100 forcontrolling charging of a battery array having a plurality of cellsbegins at a Start locus 102. The charging is effected in a plurality ofcharging modes to achieve a substantially similar target voltage in aplurality of cells. Each respective charging mode of the plurality ofcharging modes employs at least one respective charging variable toeffect the charging. Method 100 continues with the steps of, in noparticular order, establishing at least one parametric criterion foridentifying each respective cell balancing mode, as indicated by a block104; identifying at least one performance parameter associated withselected cells of the plurality of cells for each respective cellbalancing mode, as indicated by a block 106; and establishing at leastone exit criterion for permitting exiting from each respective cellbalancing mode, as indicated by a block 108. The at least one exitcriterion indicates the at least one performance parameter is within apredetermined value range for each selected cell.

[0053] Method 100 continues by posing a query whether charging is beingconducted, as indicated by a query block 110. If charging is not beingconducted, method 100 continues via NO response line 112 to return toquery block 110 via a return line 114. A delay 113 may be if desiredimposed in returning to query block 110. The optional nature of imposinga delay is indicated by delay block 113 being illustrated in dotted lineformat.

[0054] If charging is being conducted, method 100 proceeds via YESresponse line 116 to initiate cell balancing operations, as indicated bya block 118. Method 100 continues by terminating fast charging andenabling sampling of cell voltages, as indicated by a block 120. Method100 proceeds by applying the at least one parametric criterion foridentifying the respective cell balancing mode then extant; therespective cell balancing mode then extant being an extant cellbalancing mode, as indicated by a Block 121. Block 121 includes aplurality of query blocks 122, 140, 158. Method 100 poses a querywhether primary balancing is required (e.g., Region 3, FIG. 1), asindicated by a query block 122. If primary balancing is not required,method 100 proceeds via NO response line 138 to pose a query whetherconditioning balancing is required (e.g., Region 1; FIG. 1), asindicated by a query block 140. If conditioning balancing is notrequired, method 100 proceeds via NO response line 156 to pose a querywhether secondary balancing is required (e.g., Region 2; FIG. 1), asindicated by a query block 158. If secondary balancing is not required,method 100 proceeds via NO response line 172 to return via a return line133 to carry out a fast charge operation for an interval, as indicatedby a block 135. The interval during which a fast charge operation iscarried out pursuant to block 135 may be a fixed interval or theinterval may be a variable interval controlled by an operator or by acomputer program based upon predetermined decision criteria. Details ofcontrol of the interval for fast charge operations carried out pursuantto block 135 are not illustrated in FIG. 6. Method 100 proceeds fromblock 135 after the interval associated with block 135 via a line 137 toreturn to block 120 to terminate fast charging and enable furthervoltage sampling.

[0055] Once the extant cell balancing mode is ascertained (block 121)method 100 employs the at least one charging variable (preferablycharging current) for the extant cell balancing mode to effect abalancing operation appropriate to the conclusion drawn by block 121, asindicated by a block 123.

[0056] Thus, if it is determined that primary balancing is required,method 100 proceeds via YES response line 124 from query block 122 topose a query whether the primary balance algorithm is enabled, asindicated by a query block 126. If the primary balance algorithm isenabled, method 100 proceeds via YES response line 128 to effectappropriate balancing (e.g., as described in connection with FIG. 5), asindicated by a block 130. If the primary balance algorithm is notenabled, method 100 proceeds via NO response line 136 to return via areturn line 133 to carry out a fast charge operation for an interval, asindicated by block 135. The interval during which a fast chargeoperation is carried out pursuant to block 135 may be a fixed intervalor the interval may be a variable interval controlled by an operator orby a computer program based upon predetermined decision criteria.Details of control of the interval for fast charge operations carriedout pursuant to block 135 are not illustrated in FIG. 6. Method 100proceeds from block 135 after the interval associated with block 135 vialine 137 to return to block 120 to terminate fast charging and enablefurther voltage sampling.

[0057] If it is determined that conditioning balancing is required,method 100 proceeds via YES response line 142 from query block 140 topose a query whether the conditioning balance algorithm is enabled, asindicated by a query block 144. If the conditioning balance algorithm isenabled, method 100 proceeds via YES response line 146 to effectappropriate balancing (e.g., as described in connection with FIG. 3), asindicated by a block 148. If the conditioning balance algorithm is notenabled, method 100 proceeds via NO response line 154 to return viareturn line 133 to carry out a fast charge operation for an interval, asindicated by block 135. The interval during which a fast chargeoperation is carried out pursuant to block 135 may be a fixed intervalor the interval may be a variable interval controlled by an operator orby a computer program based upon predetermined decision criteria.Details of control of the interval for fast charge operations carriedout pursuant to block 135 are not illustrated in FIG. 6. Method 100proceeds from block 135 after the interval associated with block 135 vialine 137 to return to block 120 to terminate fast charging and enablefurther voltage sampling.

[0058] If it is determined that secondary balancing is required, method100 proceeds via YES response line 160 from query block 158 to pose aquery whether the secondary balance algorithm is enabled, as indicatedby a query block 162. If the secondary balance algorithm is enabled,method 100 proceeds via YES response line 164 to effect appropriatebalancing (e.g., as described in connection with FIG. 4), as indicatedby a block 166. If the secondary balance algorithm is not enabled,method 100 proceeds via NO response line 170 to return via return line133 to carry out a fast charge operation for an interval, as indicatedby block 135. The interval during which a fast charge operation iscarried out pursuant to block 135 may be a fixed interval or theinterval may be a variable interval controlled by an operator or by acomputer program based upon predetermined decision criteria. Details ofcontrol of the interval for fast charge operations carried out pursuantto block 135 are not illustrated in FIG. 6. Method 100 proceeds fromblock 135 after the interval associated with block 135 via line 137 toreturn to block 120 to terminate fast charging and enable furthervoltage sampling.

[0059] Thus, in block 123, depending upon the balancing mode employedduring charging (i.e., block 130, block 148 or block 166) for eachselected cell (preferably selected cells include all cells in thebattery array being charged) method 100 obtaining an extant parameterset; the extant parameter set being a measurement of at least one extantperformance parameter of the at least one performance parameterassociated with each the selected cell. Simply stated in the preferredembodiment of the method of the present invention, the cell voltage ismeasured as the extant parameter set. Method 100 continues, comparingeach extant parameter set with the at least one exit criterion. Method100 continues measuring the extant parameter set and comparing theextant parameter set with an appropriate exit criterion until the exitcriterion is satisfied, as described in connection with FIG. 3(conditioning balance; Region 1; FIG. 1), in connection with FIG. 4(secondary balance; Region 2; FIG. 1) and in connection with FIG. 5(primary balance; Region 3; FIG. 1).

[0060] When the appropriate exit criterion is satisfied, method 100departs block 123 to return to query block 110 via a return line 114. Adelay 113 may be if desired imposed in returning to query block 110. Theoptional nature of imposing a delay is indicated by delay block 113being illustrated in dotted line format.

[0061] Thus, if method 100 is effecting primary balance (block 130) andthe appropriate exit criterion is satisfied, method 100 returns via exitcondition line 132 to return line 114. If method 100 is effectingconditioning balance (block 148) and the appropriate exit criterion issatisfied, method 100 returns via exit condition line 150 to return line114. If method 100 is effecting secondary balance (block 166) and theappropriate exit criterion is satisfied, method 100 returns via exitcondition line 168 to return line 114.

[0062] Alternatively, a time limit may be imposed for each cellbalancing mode to establish a respective time out period as anadditional exit criterion. In such a case, if the time out periodexpires, the cell balancing operation ceases for the then extant cellbalancing mode regardless of whether the exit criterion has been met.Thus, if method 100 is effecting primary balance (block 130) and theappropriate time out period elapses, method 100 returns via time outline 134 to return line 114. If method 100 is effecting conditioningbalance (block 148) and the appropriate time out period elapses, method100 returns via time out line 152 to return line 114. In the preferredembodiment of the method of the present invention illustrated in FIG. 6,no time out parameter is provided for secondary balance (block 166)because it is in this secondary balancing operation (Region 2; FIG. 1)that balancing among cells is preferably to be carried out withoutinterruption. A time out exit criterion could, of course, beincorporated into the secondary balance aspect of method 100 if desired.

[0063]FIG. 7 is an electrical schematic diagram illustrating a firstembodiment of the cell balancing apparatus of the present invention. InFIG. 7, an apparatus 200 for controlling charging of a battery array 203is situated on a substrate 201 and is configured for connection with acharging unit (preferably located off substrate 201; not shown in FIG.7) at charger connectors 202, 204. Apparatus 200 includes a sensing unit206 and a cell balancing control unit 208. Sensing unit 206 includes acell voltage sampling device 210 coupled with battery array 203 forselectively sampling individual cells of battery array 203. Cell voltagesampling device 210 selectively converts respective cell voltages toground-referenced values. Sensing unit 206 also includes a multiplexingcontrol unit 212, a comparator 214 and a status register 216. Chargecontrol unit 208 includes a state machine 220 and a threshold generationunit 222. Threshold generation unit 222 is provided with predeterminedthreshold values TH1, TH2, THn.

[0064] Multiplexing control unit 212 is coupled with cell voltagesampling device 210 for controlling which respective cell of batteryarray 203 is sampled by cell voltage sampling device 210. An output 211from cell voltage sampling device 210 is provided to comparator 214representing each sampling of a respective cell of battery array 203.Comparator 214 also receives an appropriate threshold value TH1, TH2,THn for the then extant cell balancing mode from threshold generationunit 222. The appropriate threshold value TH1, TH2, THn is selected bythreshold generation unit 222 based upon a determination of which cellbalancing mode is extant, which is in turn based upon cell voltagesampling results selectively provided from battery array 203, as chosenby cell voltage sampling device 210, to threshold generation unit 222via a line 219.

[0065] Comparator 214 provides a comparing indication 215 to statusregister 216 indicating the result of comparing output 211 from cellvoltage sampling unit 210 with the threshold value TH1, TH2, THnreceived from threshold generation unit 222.

[0066] Apparatus 200 is an analog/digital device preferably configuredin a single-chip product. Cell voltage sampling 210 responds to signalsfrom multiplexing control unit 212 in selecting respective cells inbattery array 203 for sampling vis-à-vis a respective individualthreshold value selected for each respective cell. The first digitalmanifestation of the test and comparison with the threshold is thecomparing indication 215 provided to status register 216. Statusregister 216 provides an indication 217 to state machine 220 relatingselected comparing indications 215 received from comparator 214.Multiplexing control unit 212 is coupled with cell voltage sampling unit210, status register 216, state machine 220 and threshold generationunit 222 to ensure that the same respective cell of battery array 203 isdealt with at a given time by each of cell voltage sampling unit 210,status register 216, state machine 220 and threshold generation unit222. Multiplexing control unit 212 may be a digital unit, an analog unitor a hybrid digital-analog unit that interfaces with digital statemachine 220 and digital status registers 216. Preferably, cell voltagesampling unit 210, comparator 214 and threshold generation unit 222 areanalog devices.

[0067] State machine 220 is coupled with a shunting unit 224 forselectively effecting slow charging of selected cells in battery array203. An exemplary structure for effecting slower charging of selectedcells in battery array 203 is shown in FIG. 7: shunting unit 224establishes a current path for rerouting a portion of current aroundselected cells in battery array 203 to effect balancing among cellswhile charging. State machine 220 is also coupled with switching devicesS1, S2, S3, and controls switching action of switching devices S1, S2,S3.

[0068] In summary, apparatus 200 uses thresholds TH1, TH2, THn to detectstate of charge of respective cells of battery array 203. State machine220 controls cooperation among sensing unit 206, control unit 208 andshunting unit 224 to effect cell sampling and balancing. Cell samplingand balancing are carried out either at zero charge current or at aconditioning current. In low capacity battery arrays (i.e., batteryarrays having low charge), cell balancing takes place using apre-conditioning current. The pre-conditioning current is a low currentthat permits balancing cells in the battery array without furtherdepleting the battery array. In such conditions, cell impedance mismatcherrors are present, but they are minimized by using a lowpre-conditioning current. The minimal cell impedance mismatch errors arenot regarded as critical in this charging mode because the main cellbalancing operation is effected when the cells are at a higher capacitylevel.

[0069] In medium capacity battery arrays cell balancing is carried outat full charge current (sometimes referred to as fast charge current).The principal goal of such a cell balancing mode of operation is tominimize any cell imbalance increase that may be caused bycharge-induced offsets among the respective cells of the battery array.

[0070] In high capacity battery arrays cell balancing is carried outwith no charging current, or with a small charging current in order toachieve maximum performance and accuracy in charging operations.

[0071] Switch S2 controls current between switch S1 and battery array203. Switch S1 controls current between charger locus 202 and switch S2.Switch S3 controls whether an impedance R is included in the circuitry.Impedance R is appropriately valued so that including impedance R intothe circuitry of apparatus 200 provides a low current from charger locus202 to battery array 203 when switch S1 is open, switch S2 is closed andSwitch S3 is closed. The low current thus provided is appropriate foreffecting charging when battery array is at a low capacity.

[0072] By way of example, apparatus 200 responds to state machine 220and respective cell voltages in battery array 203 to effect chargingoperations (in cooperation with a charging device coupled to chargerloci 202, 204; not shown in FIG. 7) as indicated in Table 1 below: TABLE1 Switch S1 Switch S2 Switch S3 Low Open Closed Closed Charge CurrentSet Capacity by R Medium Closed Closed Open Charge Current Set Capacityby Charger High Open Closed Closed No Charge Current, Capacity or ChargeCurrent Set by R

[0073]FIG. 8 is an electrical schematic diagram illustrating a secondembodiment of the charging apparatus of the present invention. In FIG.8, an apparatus 300 is configured substantially the same as apparatus200 (FIG. 7) for controlling charging of a battery array 203. In theinterest of avoiding prolixity, a redundant description of apparatus 300will not be undertaken; like components in apparatus 300 and apparatus200 are identified using like reference numerals. This description ofFIG. 8 will focus upon the differences between apparatus 300 andapparatus 200 (FIG. 7). The principal difference between apparatus 300and apparatus 200 (FIG. 7) is in the provision of a current sink 350.

[0074] Current sink 350 is configured to ensure that a predeterminedcurrent continues to flow from a charger unit even when a low chargecurrent condition (low capacity) or a no charge current condition (highcapacity) exists in the battery array being charged. Current sink 350may be embodied in any circuitry that selectively shunts a predeterminedcurrent to ground when apparatus 300 is effecting current balancingoperations requiring certain levels of charging current. In thepreferred embodiment of apparatus 300, the low current level requiredfor effecting charging of a low capacity battery array 203 is providedby the charging unit itself, and the no current operation isaccommodated by current sink 350. In the preferred embodiment, currentsink 350 provides a predetermined current draw from the charging unitsufficient that the charging unit does not detect a no current situationand prematurely erroneously terminate charging operations.

[0075] The preferred embodiment of current sink 350 is illustrated inFIG. 8 in dotted line format. A switch S8 responsive to state machine220 selectively includes an impedance R8 within the circuitry ofapparatus 300 to shunt current to ground from the charging unit (notshown in FIG. 8) coupled with charging loci 202, 204. Impedance R8 isselected appropriately to ensure that sufficient current is drawn fromthe charging unit to preclude inappropriate premature termination ofcharging operations by the charging unit. No communication is requiredbetween the host unit being charged (not shown in FIG. 8) or betweenapparatus 300 and the charging unit. The inclusion of current sink 350within the circuitry of apparatus 300 is effected in response to thecell balancing mode selected for charging battery array 203 based uponcomparison of cell voltages with thresholds TH1, TH2, THn, as describedearlier herein in connection with FIGS. 1-7.

[0076] In summary, apparatus 300 uses fixed thresholds TH1, TH2, THn todetect state of charge of battery array 203. State machine 220 controlscooperation among sensing unit 206, control unit 208 and shunting unit224 to effect cell sampling and balancing. Cell sampling and balancingare carried out either at zero charge current or at a conditioningcurrent. In low capacity battery arrays (i.e., battery arrays having lowcharge), cell balancing takes place using a pre-conditioning current.The pre-conditioning current is a low current set by the charging unit(not shown in FIG. 8) that permits balancing cells in the battery arraywithout further depleting the battery array. In such conditions, cellimpedance mismatch errors are present, but they are minimized by using alow pre-conditioning current. The minimal cell impedance mismatch errorsare not regarded as critical in this cell balancing mode because themain cell balancing operation is effected when the cells are at a highercapacity level.

[0077] In medium capacity battery arrays cell balancing is carried outat full charge current. The principal goal of such a cell balancing modeof operation is to minimize any cell imbalance increase that may becaused by charge-induced offsets among the respective cells of thebattery array.

[0078] In high capacity battery arrays cell balancing is preferablycarried out with a small charging current established by current sink350 in order to achieve maximum performance and accuracy in chargingoperations. Current sink 350 preferably ensures that a sufficient loadis “seen” by a charger unit (not shown in FIG. 8). This avoidsinterruption of charging operations which might otherwise occur if thecharger unit erroneously “saw” a too-low load and interpreted thattoo-low load as indicating a completion of charging.

[0079] Switch S2 controls current between switch S1 and battery array203. Switch S1 controls current between charger locus 202 and switch S2.Switch S8 controls whether an impedance R8 is included in the circuitryto draw a predetermined current from the charging unit and shunt toground. Impedance R8 is appropriately valued so that including impedanceR8 into the circuitry of apparatus 300 provides a low current fromcharger locus 202 to ground when switch S1 is open, switch S2 is closedand switch S8 is closed. The low current thus drawn from charging locus202 is appropriate to preclude premature erroneous termination ofcharging operations by the charging unit.

[0080] By way of example, apparatus 300 responds to state machine 220and respective cell voltages in battery array 203 to effect chargingoperations (in cooperation with a charging device coupled to chargerloci 202, 204; not shown in FIG. 8) as indicated in Table 2 below: TABLE2 Switch S1 Switch S2 Switch S8 Low Closed Closed Open Charge CurrentSet Capacity by Charger Medium Closed Closed Open Charge Current SetCapacity by Charger High Open Closed Closed No Charge Current; CapacityPredetermined Current Drawn From Charger

[0081] Apparatus 300 does not require communication between apparatus300 (or a host product for which apparatus 300 is controlling cellbalancing operations) and the charger unit (not shown in FIG. 8) coupledwith charger loci 202, 204. The charger unit does not requireinformation regarding when low current charging operations or no currentcharging operations are being carried out. The inclusion of current sink350 within the circuitry of apparatus 300 is effected in response to thecell balancing mode selected for charging battery array 203 based uponcomparison of cell voltages with thresholds TH1, TH2, THn, as describedearlier herein in connection with FIGS. 1-7.

[0082] It is to be understood that, while the detailed drawings andspecific examples given describe preferred embodiments of the invention,they are for the purpose of illustration only, that the apparatus andmethod of the invention are not limited to the precise details andconditions disclosed and that various changes may be made thereinwithout departing from the spirit of the invention which is defined bythe following claims:

I claim:
 1. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit; the apparatus comprising a current sink switchingly coupled with said input locus for selectively contributing a predetermined current draw at said input locus.
 2. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit as recited in claim 1 wherein said current sink comprises at least one impedance device.
 3. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit as recited in claim 2 wherein said at least one impedance device is arranged for said switching coupling in at least one impedance unit; each said at least one impedance unit including at least one said impedance device.
 4. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit as recited in claim 1 wherein said at least one impedance device is at least one resistor.
 5. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit as recited in claim 2 wherein said at least one impedance device is at least one resistor.
 6. An apparatus for use with a charge control system for affecting current draw from a charging unit coupled at an input locus of said charge control system for charging a battery unit as recited in claim 3 wherein said at least one impedance device is at least one resistor.
 7. An apparatus for selectively establishing a predetermined current draw from a charging unit coupled with a charge control unit for charging a battery unit; said charging unit being coupled with said charge control unit at an input locus; the apparatus comprising at least one resistor and at least one switch unit coupled between said input locus and ground; said at least one switch unit opening and closing in response to said charge control unit to establish said predetermined current draw when predetermined conditions are sensed in said battery unit.
 8. An apparatus for selectively establishing a predetermined current draw from a charging unit coupled with a charge control unit for charging a battery unit as recited in claim 7 wherein said at least one resistor comprises one resistor.
 9. An apparatus for selectively establishing a predetermined current draw from a charging unit coupled with a charge control unit for charging a battery unit as recited in claim 7 wherein said at least one resistor and said at least one switch are arranged for switchingly coupling at least one impedance unit between said input locus and ground; each said at least one impedance unit including at least one said at least one resistor.
 10. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit; the method comprising the steps of: (a) providing a current sink switchingly coupled with said input locus; (b) sensing at least one predetermined condition in said battery unit; and (c) switchingly engaging said current sink when said at least one predetermined condition satisfies at least one predetermined criteria.
 11. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit as recited in claim 10 wherein said current sink comprises at least one impedance device.
 12. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit as recited in claim 11 wherein said at least one impedance device is arranged for said switchingly engaging at least one impedance unit; each said at least one impedance unit including at least one said impedance device.
 13. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit as recited in claim 10 wherein said at least one impedance device is at least one resistor.
 14. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit as recited in claim 11 wherein said at least one impedance device is at least one resistor.
 15. A method for selectively establishing a predetermined current draw from a charging unit coupled at an input locus with a charge control unit for charging a battery unit as recited in claim 12 wherein said at least one impedance device is at least one resistor. 